Author: Walter Faxon
Date: 21:49:11 12/05/02
Go up one level in this thread
On December 02, 2002 at 20:27:39, Gerd Isenberg wrote: >Ahh.. i see, > >a special kind of popcount for consecutive trailing "ones". >I love this interactive programming lessions. > >Gerd > >Btw.: Impressive assembler listing: > >00401003 8B 4C 24 0C mov ecx,dword ptr [esp+0Ch] >00401007 8B 31 mov esi,dword ptr [ecx] >00401009 8B 79 04 mov edi,dword ptr [ecx+4] >0040100C 8B D6 mov edx,esi >0040100E 8B C7 mov eax,edi >00401010 83 C2 FF add edx,0FFFFFFFFh >00401013 83 D0 FF adc eax,0FFFFFFFFh >00401016 23 F2 and esi,edx >00401018 23 F8 and edi,eax >0040101A 89 31 mov dword ptr [ecx],esi >0040101C 89 79 04 mov dword ptr [ecx+4],edi >0040101F 8B CF mov ecx,edi >00401021 33 D6 xor edx,esi >00401023 33 C1 xor eax,ecx >00401025 33 C2 xor eax,edx >00401027 5F pop edi >00401028 35 81 FC C5 01 xor eax,1C5FC81h >0040102D 5E pop esi >0040102E 8B D0 mov edx,eax >00401030 C1 EA 10 shr edx,10h >00401033 03 C2 add eax,edx >00401035 33 D2 xor edx,edx >00401037 8B C8 mov ecx,eax >00401039 C1 E9 08 shr ecx,8 >0040103C 2A C1 sub al,cl >0040103E 25 FF 00 00 00 and eax,0FFh >00401043 8A 90 D5 62 40 00 mov dl,byte ptr [eax+4062D5h] >00401049 8B C2 mov eax,edx >0040104B C3 ret Hi, Gerd. I have noticed that most compilers aren't too smart where it comes to bitwise dataflow analysis. After the "add eax, edx" instruction above, better (well, shorter!) code would be: sub al, ah and eax, 0FFh mov al, byte ptr [eax+4062D5h] ret Perhaps if we added a little "hint" in the last 2 lines of source: inline // inline declaration may differ by compiler u8 LSB_64( u64* bb ) { u64 t64; u32 t32; u8 t8; t64 = *bb - 1; *bb &= t64; // omit this line to retain current LSB t64 ^= *bb; t32 = (u32)t64 ^ (u32)(t64 >> 32); t32 ^= LSB_64_magic; t32 += t32 >> 16; t8 = (u8)t32 - (u8)(t32 >> 8); return LSB_64_table [LSB_64_adj + t8]; } Of course, this really only helps because we already know that the x86 class can independently address both the first and 2nd bytes of a register. And it's wishful thinking that much better asm code will actually be produced! I'd be interested to see. If/when you run your test suite again, please try this version. Thanks. -- Walter
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